Leakage in MOS devices

1. Leakage in MOS devices Mohammad Sharifkhani

2. Reading • Text book, Chapter III • K. Roy’s Proc. of IEEE paper

3. Introduction • What is leakage? • IOFF (drain current when transistor is supposed to be off) • Including gate leakage • Why is it important? • Stand-by power; energy consumption for no work

4. Introduction • How bad is it? • 1nA/um @0.25um @30 degree C • 1uA/um @0.1um @80 degree C • Each generation for a 15mm2 chip • I off increase by 5x • Total Width increase by 50% •  Total leakage current on a chip 7.5x •  Leakage power 5x

5. Introduction

6. MOS Leakage behavior

7. Leakage components • 6 leakage components • I1: PN junction reversed bias • I2: Subthreshold leakage • I3: Gate tunneling • I4: Hot carrier injection • I5: GIDL • I6: Punchthrough

8. PN junction reverse bias current • Minority carrier drift/diffusion • Near the edge of depletion region • The direct band-to-band tunnelling model (BTBT) • Describes the carrier generation in the high field region without any influence of local traps. • Electron-hole generation in depletion region • Band to band tunneling (BTBT) is dominant

9. PN junction reverse bias current • Tunneling current density increases exponentially with doping: • Na, Nd • Vapp (drops too, minor effect) • Doping increases with scaling • For typical devices it is between 10pA – 500pA at room temperature; For a die with million devicesoperated at 5 V, this results in 0.5mW power consumption  rather small • For 0.25 μm CMOS: J = 10-100 pA/ μm2 at 25 deg C.

10. Subthreshold leakage • Most important among all • Weak inversion • Minority carriers in the channel is small but not zero • Small Vds; drops across the reversed-bias pn; small field • small field, carrier  current is due to diffusion rather than drift (base in BJT) • Wdm: maximum width of depletion layer; m<2

11. Subthreshold leakage • When Vth is small  Vgs = 0 does not turn ‘off’ the MOS

12. Subthreshold leakage • Exponential relationship with Vgs and Vth • 255mV Vth variation  3 orders of magnitude in leakage • St; milivolts/decade • Threshold voltage variation effect on leakage • About 70-120mV/dec • Smaller St: sharper slope • Less voltage variation for 10x leakage increase

13. Subthreshold leakage (DIBL) • Drain Induced Barrier Lowering • Short channel devices • Depletion region of drain interacts with source near channel surface • Voltage at the drain lowers the potential barrier at the source • Lowers VTh • Increases subthreshold current without any change onS • Causes source to inject carriers into channel surface independent of the gate voltage • More DIBL at higher VD and shorter Leff • Moves curve up, to right, as VD increases

14. Subthreshold leakage (Body Effect) • Vth roll off • Increase of Vth with reduction of Channel Length • Reverse body bias • Widens depletion region • Length ↓, Vth↑ • Bulk doping ↑  Vth substrate sensitivity ↑ • Reverse body bias ↑  Vth substrate sensitivity ↓ • Slope St remains the same

15. Subthreshold leakage (Narrow Width Effect) • Isolations • Local Oxide Isolation (LOCOS) • Trench isolation • In LOCOS, the fringing field causes the gate-induced depletion region to spread outside the channel width and under the isolations • Gate has to work more to create the channel (inversion) • More substantial (comparable) as the channel width decreases •  Increase of Vth due to narrow-channel effect • Kicks in for W<0.5um

16. Subthreshold leakage (Narro Width Effect) • Trench isolated technologies: – Vt decreases for effective channel widths W ≤ 0.5 μm NMOS • For PMOS: A much more complex behavior • reduction of the width first decreases the until the width is 0.4 m. The width reduction below 0.4 um causes a sharp increase

17. Subthreshold leakage (Channel Length Effect) • Short-channel devices: source-to-drain distance comparable to depletion width in vertical direction • Source and drain depletion regions penetrate more into channel length. • Part of the channel being already depleted. •  Gate voltage has to invert less bulk charge to turn a transistor on.

18. Subthreshold leakage (Temperature Effect) • 23 fA/um to 8 pA/um • Factor of 356 • Smaller St: • Sharper transition (worse sensitivity) • Two parameters increase the subthreshold leakage as temperature is raised: • 1) Vth linearly increases with temperature • 2) the threshold voltage decreases. • The temperature sensitivity of was measured to be about 0.8 mV C.

19. Gate Leakage • Tox ↓  Eox ↑ • Two mechanisms of electron tunneling • Fowler–Nordheim Tunneling: electrons tunnel into conduction band of oxide layer • Very high field strength; usually not present in products • Direct Tunneling: electrons from the inverted silicon surface to the gate through the forbidden energy gap of the SiO2 layer

20. Hot Carrier Injection • In a short-channel transistor, due to high electric field near the Si–SiO2 interface, electrons or holes can gain sufficient energy from the electric field to cross the interface potential barrier and enter into the oxide layer • Reliability risk! (Electrons can trap into or destroy oxide) • Increases as L drops (unless VDD drops accordingly)

21. Gate-Induced Drain Leakage (GIDL) • GIDL is due to high field effect in the drain junction of an MOS transistor • Vg<0  Thins out the depletion region between drain to well PN junction • Effect of new electric field on the old PN depletion region  holes tunnel to substrate from drain • Since the substrate is at a lower potential for minority carriers, the minority carriers that have been accumulated or formed at the drain depletion region underneath the gate are swept laterally to the substrate, completing a path for the GIDL - - - - - + - + + + +

22. Gate-Induced Drain Leakage (GIDL) • The effect of GIDL is more visible at higher VDD and lower Vg • Thinner oxide thickness and higher VDD (higher potential between gate and drain) enhance the electric field and therefore increase GIDL • Increase from 4nA  36nA (for VD from 2.7V to 4V)

23. Gate-Induced Drain Leakage (GIDL) • Increasing current for negative VG values • • Localized along channel width between gate and drain • • Major problem in Ioff current: • • Contributes to standby power, so must control this by • increasing oxide thickness, increasing drain doping, or • eliminating traps. • • For high performance device (low Vth), is not a major • issue.

24. Punchthrough • When Source and Drain depletion region “touch” each other deep in the channel. •  Less gate influence on the current • Channel is created deeper in substrate • Higher St • Varies quadratically with VD and with VS

25. Leakage component contribution • In each region, the last term dominates